1. Field of the Invention
This invention relates to a device for optically measuring plasma properties.
2. Related Art Statement
MHD power generation, that is, power generation by the use of magnetohydrodynamics, as a new technology for generating electricity, has the great advantage of directly converting the energy of a high-temperature fluid into electrical energy by flowing high-temperature fuel gas plasma in a direction perpendicular to a magnetic field. The electromotive forces of the MHD power generation are induced by the Faraday effect in two directions, i.e., in a direction perpendicular to the magnetic field and in a direction perpendicular to the plasma flow. It is necessary in MHD power generation to maintain the high-temperature plasma under desired conditions. To this end, there is a strong demand for developing a device which accurately measures the temperature, electron density, conductivity and other physical properties of the plasma.
There is a conventional method for optically measuring plasma temperature based on the so-called line reversal principle. According to the line reversal principle, a reference light source projects light flux of various light source temperatures onto a plasma, and one detects that light source temperature at which the light flux incident on the plasma is not absorbed by the plasma and plasma emission does not cause any increase of light intensity. Such light source temperature is treated as being equal to the plasma temperature, whereby the plasma temperature is detected. Methods based on the line reversal principle can measure a plasma temperature of up to about 3,000.degree. K., and such methods include a wavelength sweeping method, a matching method, a chopper method, and a knife edge method.
FIG. 5 shows a schematic diagram of a device for measuring the plasma temperature by the wavelength sweeping method. A light source 1 of white light is connected to a reference power source 2, and light fluxes of various light source temperatures are radiated from the light source 1 by changing the output from the reference power source 2 and such light fluxes are projected onto a plasma 4 through a condenser lens 3. The light fluxes which have passed the plasma 4 become incident on a spectrophotometer 7 through a slit 5 and another condenser lens 6, and the spectrographic property of such light fluxes through the plasma 4 are detected. The detected spectrographic properties are stored in a recorder 9 through an amplifier 8. Depending on the light source temperature T.sub.L at the light source 1, the light fluxes through the plasma 4 are affected by light emission or absorption in the plasma 4 in the resonant wavelength range as shown in FIG. 6. More particularly, when the light source temperature T.sub.L is below the plasma temperature Tp, the light intensity is increased by the light emission in the plasma in the proximity of the resonant wavelength, while when the light source temperature T.sub.L is above the plasma temperature Tp, light absorption is caused. When the light source temperature T.sub.L is equal to the plasma temperature Tp, neither light emission nor light absorption is caused. Accordingly, if the spectrographic light intensity characteristics of the light fluxes through the plasma 4 is measured while varying the light source temperature of the white light source 1, the plasma temperature Tp can be determined.
FIG. 7 shows a schematic diagram of a device for measuring the plasma temperature by the knife edge method. In the knife edge method, two condenser lenses 10 and 11 are disposed between the light source 1 and the plasma 4, and a knife 12 is disposed between the two condenser lenses 10 and 11. Whereby, due to the presence of the knife 12, only those light fluxes which pass below an optical axis shown on the figure are allowed to become incident on the plasma 4. The light fluxes which leave the plasma 4 pass through a condenser lens 13, a slit 14 and a filter 15, and enter into light guides 16 and 17, which light guides are disposed above and below the optical axis. Light detectors 18 and 19 are disposed on the output sides of the light guides 16 and 17, and the outputs from the light detectors 18 and 19 are stored in recorders 20 and 21 respectively. Those light fluxes from the light source 1 which come into the space above the optical axis are interrupted by the knife 12, so that the light detector 16 located above the optical axis detects only the light fluxes radiated by the plasma radiation field, while the light detector 17 located below the optical axis detects the sum of the flux from the light source 1 and the fluxes radiated from the plasma radiation field. Thus, the plasma temperature Tp is measured based on the output signals from the light detectors 16 and 17.
The above-mentioned wavelength sweeping method has a shortcoming in that, since the light source temperature of the white light source is required to be successively changed in order to allow the measurement of the spectrographic properties at each light source temperature, it takes a long time to complete the measurement, and the conditions of the plasma are changed during the measurement, so that the plasma temperature of one spatial point at a specific time can not be determined. It has another shortcoming in that the plasma temperature can be measured only at discrete points, and continuous measurement with respect to time is impossible. Further, theoretically speaking, only the approximate temperature is measured and the reliability of the measured value is not very high.
The knife edge method also has a shortcoming in that there is no spatial coincidence between the light fluxes coming from the light source through the plasma and the light fluxes radiated from the plasma radiation field, so that a serious measurement error may be caused when there is any significant localized variation of plasma conditions.
The above shortcomings are noticed not only in the plasma measurement but also in other measurement of quickly varying physical phenomenon such a nuclear fusion reaction and an electric discharge.